The expansion centers on the Lovecraft-themed creatures in the Warcraft universe. It will add 134 new cards to the game. Blizzard says:

These are no ordinary cards! Steeped in the insidious influence of the Old Gods, unknown terrors will give your decks new and bizarre forms. You may even recognize some old friends that have succumbed to the seductive promise of Ancient Cosmic Power.

Players can pre-order a 50 card pack for Whispers of the Old Gods starting March 14 for $49.99. Those folks will also gain access to an exclusive card back as a bonus. You can download Hearthstone now from the iOS app store.